A significant portion of the product design cycle of many modern manufactured products has generally been consumed by the time required to build and test prototype parts. Accordingly, many manufacturers operate fast turn-around shops for the producing of prototype parts useful in the development of the manufactured product. One popular method of producing prototype parts in such shops is the subtractive machining of a block of material until the part matches the dimensions in a mechanical design. As is well known, the accuracy with which the machined prototype part matches the design can widely vary, primarily according to the skill of the machinist. In addition, the ability of subtractive processing to produce parts of complex shape is limited, and the time required for the machining of the part can be quite lengthy. As such, the product design cycle depending upon subtractive machining of prototypes is often lengthy, delaying the time-to-market of the eventual manufactured product.
Accordingly, new methods for the producing of parts, especially prototypes, have been developed in recent years to enable the rapid manufacture of complex parts directly from computer-aided-design (CAD) data bases. In particular, additive processes for building up the parts from a material have recently become popular, such processes in contrast to subtractive processes which remove material from a block to form the part. One type of such an additive process is stereolithography, as described in U.S. Pat. Nos. 4,575,330 and 4,929,402. Other processes for producing parts also selectively photopolymerize liquids, as described in U.S. Pat. Nos. 5,031,120 and 4,961,154. Additive processes by which parts are produced by jetting of droplets of solidifiable material are described in U.S. Pat. Nos. 5,059,266 and 4,665,492.
By way of further background, an additive powder-based process is described in Sachs, et al., "Three Dimensional Printing of Ceramic Shells and Cores for Metal Casting", presented at the 39th Annual Technical Meeting of the Investment Casting Institute (1991), pages 12:1 through 12:13. In the process described in this article, a part is produced in layerwise fashion from a powder. After the deposition of a layer of the powder, a binder is applied to selected locations of the layer to form a cross-section of the part, using technology similar to ink-jet printing. Additional layers of the powder are spread and similarly processed to build up the part, after which the unbound powder is removed to leave the part.
By way of further background, another additive process is described in Weiss, et al., "A Framework for Thermal Spray Shape Deposition: The MD.sup.* System", presented at the Solid Freeform Fabrication symposium (The University of Texas at Austin, August, 1991). This additive process utilizes thermal or plasma spraying of material, such as metals, through a mask to deposit a cross-section of a part. Support material (such as a lower melting point metal) is thermally sprayed through a complementary mask to fill in the layer, allowing overhangs of the primary material to be formed in subsequent layers.
A particularly successful and recently developed additive process is commonly referred to as selective laser sintering. According to the selective laser sintering process, a laser is scanned in raster fashion over a layer of fusible powder to fuse selected portions of the layer according to a cross-section of the desired part. After the fusing of the desired portions of a layer, another layer of powder is placed and similarly selectively fused, with fused portions of the later layer fusing to fused portions of the previous layer. Continued layerwise processing in this manner results in a part which can be quite complex in the three-dimensional sense. This method is described in detail in copending application Ser. No. 814,715, filed Dec. 30, 1991, now abandoned and in the above-referenced U.S. Pat. No. 5,076,869, issued Dec. 31, 1991, and U.S. Pat. No. 4,944,817 issued Jul. 30, 1990, all assigned to Board of Regents, The University of Texas System, and incorporated herein by this reference. The selective laser sintering method is also described in U.S. Pat. No. 4,863,538, issued Sep. 5, 1989, U.S. Pat. No. 5,017,753 issued May 21, 1991, and U.S. Pat. No. 4,938,816 issued Jul. 3, 1990, all also assigned to Board of Regents, The University of Texas System and incorporated herein by this reference.
By way of further background, copending applications Ser. No. 624,419 filed Dec. 7, 1990, now U.S. Pat. No. 5,156,697 issued on Oct. 20, 1992, Ser. No. 657,151 filed Feb. 19, 1991, and Ser. No. 692,172, filed Apr. 26, 1991, now U.S. Pat. No. 5,147,587 issued on Sep. 15, 1992, all also assigned to Board of Regents, The University of Texas System and incorporated herein by this reference, as well as the other referenced U.S. Patents noted hereinabove, each describe the selective laser sintering of various materials and combinations of materials such as plastics, waxes, metals, ceramics, and the like. In particular, the selective laser sintering method has been especially beneficial in the production of molds or cores useful in investment casting. For example, a part formed of a low-temperature wax by the selective laser sintering process may be used in the well-known "lost wax" method of forming an investment casting mold. In addition, the above-referenced copending applications Ser. No. 624,419, now U.S. Pat. No. 5,156,697 issued on Oct. 20, 1992 Ser. No. 657,151, now U.S. Pat. No. 5,147,587 issued on Sep. 15, 1992, and Ser. No. 692,172 now U.S. Pat. No. 5,182,170 issued on Jan. 26, 1993, each describe methods for producing parts from high temperature materials, and thus which may be useful in directly forming an investment casting mold.
In the selective laser sintering process, sufficient energy must be directed to the powder so as to cause it to fuse into the desired part cross-section. For most powders processed in this manner, including wax, plastic and metallic powders, the fusing mechanism is sintering, in which the surface tension of the irradiated powder overcomes its viscosity, such that the particles flow together and bond. As such, the temperature at which sintering occurs is substantially the melting or softening point of the powder material. For waxes and plastics, the melting point can be sufficiently low so that a low power laser (e.g., a 100 watt NdYAG laser) can sinter the material. For higher temperature materials, either higher power lasers must be used, or the temperature of the chamber in which the selective laser sintering process is carried out must be raised to near the sintering temperature. Accordingly, the production of parts of high melting point materials, such as ceramics useful as investment casting molds, is significantly more difficult than such production of lower melting point materials.
In addition, thermal gradient-related effects such as warpage and shrinkage must also be controlled in the selective laser sintering process, particularly for high temperature materials. Warpage has been observed in parts where a bottom flat surface curls up at the edges to become a curved surface, concave up. It is believed that such warpage is due to the thermal shrinkage of the sintered layer from its temperature during sintering to its post-sintering temperature, and, in some cases, the reduction in volume of a layer as it passes through the phase change from liquid to solid. The reduction in volume of a newly sintered layer, whether by phase change or by a drop in temperature, causes the top of the part to contract. The bottom of the part is thermally insulated by its immersion in unsintered powder and in previously sintered layers that have already contracted; as a result, contraction of the top layer induces stress that can curl the part. Furthermore, uneven cooling of the part during its layer-wise manufacture, for example where top layers of the part are cooled more quickly than bottom layers, has also been observed to cause warpage and curling.
Also as noted hereinabove, some materials tend to shrink in the consolidation from powder to a high density solid that occurs in the sintering process. Particularly for high temperature materials, such shrinkage causes undesirable loss of dimensional accuracy.
Because of these thermal effects, it is therefore desirable to perform the selective laser sintering at relatively low temperatures, so that the thermal gradients present in the system are controlled. However, such low temperature processing is not compatible with the desire to form high strength, high temperature parts, such as molds or cores for investment casting.
By way of further background, another method for producing parts of high temperature materials, such as high temperature ceramics and ceramic composites, utilizes a powder of polymer-coated ceramic, such as described in the above-referenced U.S. Pat. Nos. 5,076,869 and 4,944,817. As described in these Patents, after the formation of a part by flowing the polymer coating to bind particles of a high temperature material, the part is subjected to a post-process anneal, in which the polymer coating dissociates and the remaining particles of the high-temperature material sinter together and form the part. However, it has been observed that the particle sizes necessary for the selective laser sintering process can be too large to subsequently sinter in the post-process anneal, at least in a reasonable anneal time. In addition, the sintering temperature of important ceramic materials such as alumina and silica often exceeds the temperature available in conventional ovens.
It is therefore an object of the present invention to provide a method of producing parts having high melting points, by way of a low power energy beam.
It is a further object of the present invention to provide such a method in which the parts may be produced with a high degree of dimensional tolerance.
It is a further object of the present invention to provide such a method in which the ceramic, metal and metal/ceramic parts may be produced.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.